Abstract These studies propose a detailed investigation of the structural and functional consequences of arenavirus neutralizing antibody binding. The analysis of monoclonal antibodies developed for arenaviruses has shown that neutralizing epitopes localize to the viral glycoprotein (GP-C), particularly to the receptor-binding GP-1 subunit (8). Earlier studies have defined epitope regions on GP-C, and identified several cross-reactive antibodies. Binding sites of LCMV antibodies will be used as a paradigm to define GP-1 neutralizing epitopes across the arenaviridae. Novel antibodies could have therapeutic, diagnostic, or disease response potential, since protective vertical or humoral transfer of passive immunity has been reported in animal models of arenavirus infection. Junin virus, the agent of Argentinean Hemorrhagic fever, has also been shown to be amenable to treatment by intervention with human convalescent sera. Neutralizing antibody epitopes will be mapped to sequences and the low-resolution 3D cryo-EM structure of GP-C. Effects of neutralizing antibody binding on viral attachment, fusion and infectivity will be investigated separately. These results will be applied to the design of new antibodies with the highest probability of neutralizing activity for Junin virus, based on the high percentage of amino acid conservation among arenaviruses. Antibodies will be produced, targeted to regions of the Junin virus and vaccine strain Candid 1 GP-C that are homologous to regions containing LCMV neutralizing epitopes. The neutralizing potential of these antibodies will be compared to known and novel Junin antisera and serum from vaccinated or convalescent Junin patients to determine how GP-C epitopes and functions are targeted in a protective humoral response. The overall aim is to develop a rational structure-based means of designing neutralizing antibodies to any known or emerging arenavirus.